Astronauts Set for Spacewalk Thursday to Fix Ammonia Leak

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Two astronauts will venture outside the International Space
Station Thursday (Nov. 1) to isolate and repair an ammonia leak
in part of the orbiting lab's cooling system.

NASA astronaut
Sunita Williams and Japanese spaceflyer Akihiko Hoshide are
slated to start the spacewalk — also known as an extra-vehicular
activity, or EVA — Thursday at 8:15 a.m. EDT (1215 GMT). They're
expected to stay outside for about 6 1/2 hours, NASA officials
said. You can watch live coverage of the spacewalk
here on NASA TV, beginning at 7:15 a.m. EDIT (1115 GMT).

Each of the orbiting lab's eight huge solar arrays has its own
associated power system, and circulating ammonia helps cool this
gear down. The spacewalk's first objective is to find the source
of the leak, which could affect one of these power channels if
it's not addressed soon.

"We don't know exactly where the leak is," space station manager
Mike Suffredini of NASA told reporters during a teleconference
last Friday (Oct. 26). "It's possible the leak is in the PVR
itself, the [photo-voltaic] radiator itself. It could be in the
pump system, or it could be in any one of the [coolant] lines."
[ Gallery:
Building the International Space Station ]

On Thursday's EVA, Williams — commander of the station's current
Expedition 33 mission — and Hoshide will make their way over to
the port side of the station's backbone-like truss. They'll
reconfigure some lines in the affected coolant system and install
a spare radiator to see if that stops the leak.

"We're real suspicious of the radiator," said spacewalk flight
director Mike Lammers, of NASA's Johnson Space Center in Houston.
"It kind of stretches out there and is susceptible to
micrometeorite impacts."

Because the leak is so slow, it will likely take several weeks
before it's known if the fix works, officials said.

If ammonia continues to escape, the station's operators will
eventually have to try something else. For example, they may
direct astronauts to swap out some of the coolant system's pump
gear on another spacewalk. But there likely won't be an urgent
need for a new fix, since the spare radiator should provide
enough ammonia to keep the coolant system operating until next
October or so, officials said.

NASA and its space station partners have known about the ammonia
leak since 2007, and they've taken measures to deal with it
before. Astronauts aboard the
space shuttle Endeavour recharged the coolant system with 8
pounds (3.6 kilograms) of ammonia last year.

Spacewalks are always challenging, but Thursday's EVA is unlikely
to throw anything at Williams and Hoshide that the two astronauts
can't handle. In late August and early September, the duo
replaced a vital power unit over the course of two
spacewalks, defeating an unexpectedly stubborn stuck bolt in the
process.

"I would say this is an average-skill-level EVA — maybe slightly
more challenging than an average EVA," said lead spacewalk
officer Allison Bolinger of the Johnson Space Center.